Psoralens (furocoumarins) are potent photosensitizing agents for eliciting skin erythema and skin cancer. The molecular mechanism of the psoralen photosensitization is likely to involve photochemical modifications of DNA via covalent photo-coupling of psoralens to pyrimidine bases. The structures of psoralen-pyrimidine photoadducts have not been chemically established due to lack of a method for preparative scale production of the photoadducts and the diversity of a number of different photoproducts. We propose to study the photochemistry of dimethoxycoumarin-pyrimidine systems as a simple model in order to establish the structural aspect of the photoadducts relevant to the skin sensitizer-DNA photoadditon process; in particular, structural elucidation of the cycloaddition between the excited DMC and pyrimidine base will be studied, since DMC is expected to yield more well-defined (in terms of a number of different products) photoadducts at relatively high yields than psoralens for chemical structure and stereochemical investigations. DMC as a model photosensitizer is also biologically relevant, since it is the only one of the many coumarin derivatives which shows a significant photolethal effect in Bacillus subtilis mutants. Preliminary results also indicate that DMC intercalates and photochemically cross-links to DNA, contrary to other coumarins.